The invention concerns a process and apparatus for the purification of chemical substances, which initially are melted and crystallized by the removal of heat, whereupon they are exposed to a sweating process in which the contaminated crystallized surfaces are melted and the molten part removed prior to the recovery of the remaining purified substance.
It is known (e.g., see Chemie-Ingenieur-Technik 57 (1985), pg. 97) to carry out the purification of chemical substances, for example of acetic acid, by melt crystallization in tubular type crystallizers. This process is based upon the fact that crystals are chemically purer than the initial melt. Impurities are pressured against grain boundaries in the course of crystal formation or remain in the residual melt.
The known processes operate discontinuously. The melt crystallizes in a first stage on cooled walls. Prior to the remelting of the now pure crystalline layer, a so-called sweating is carried out. By raising the temperature just above the melting point, the crystalline surface, still contaminated, is melted. An outer portion of the product sweats-off and runs down so as to be collected and removed in the form of a melt. By further raising the temperature, the purified crystalline layer itself is melted, thereby obtaining the final substance.
It is an object of the present invention to provide a process and apparatus whereby a purification process may be carried out continuously.